1. Field of the Invention
The present invention relates to laminated packaging materials including a gas barrier layer, which comprises starch or a starch derivative, and to methods of making such laminates. The invention also relates to packaging containers and packages manufactured from the laminated packaging materials of the invention.
2. Description of the Related Art
In the packaging industry, use is often made of packages of the single-use type for packaging and transporting of various products, such as, for example, liquid food products. In order to benefit from respective advantageous properties of various materials, such different materials are often laminated together into a packaging laminate. Depending on circumstances, the aim is often to provide the best possible protection to the product to be packed, at the same time as the package must be sufficiently mechanically strong to allow for convenient handling.
For packaging of liquid food products during prolonged transport and storage, it may be important that the packaging laminate provides barrier properties towards gases, such as oxygen as well as towards liquids and moisture. Usually, the package is also required to be dimensionally stable in order to provide easy handling at transport and distribution as well as in use.
Liquid barrier properties are usually provided in a laminated packaging material by means of outer layers of plastics. Such outer layers of plastics are usually also sealable to each other by means of melt fusion, in order to provide liquid tight seals of the package in a cost efficient and reliable manner. Normally thermoplastics, such as polyolefins are employed for this purpose, and polyethylenes, such as preferably low density polyethylenes (LDPE), are most often used.
Mechanical strength and configuration stability may be obtained by the incorporation of a more rigid, core layer in the packaging laminate, usually a dimensionally stable but foldable paper or paperboard. This is usually an intermediate layer but the term xe2x80x9ccore layerxe2x80x9d used herein includes such layers even when they form one external face of the laminate.
Barrier properties towards oxygen gas are obtained by incorporation of a layer of one of a number of barrier materials known in the art. Examples of such known oxygen barrier materials are metals and metal oxides such as aluminium foil (Al foil), silicon oxide and siloxane coatings (SiOx) on to a polymer substrate layer as well as polymer materials having gas barrier properties, such as polyamides (PA), polyvinyl alcohol (PVOH), ethylene vinyl alcohol polymers (EVOH) and polyehtyleneterephtalate (PET). In addition to such synthetic materials, the possibility of using natural and biodegradable polymers (biopolymers) such as starch and starch derivatives, as gas barrier materials has been investigated.
It is previously known that starch possesses some gas barrier properties when employed in relatively thick layers, such as in films having a thickness of about 20 to 30 xcexcm. Such thick layers of starch material are not suitable for use in packaging laminates however, since they become brittle and are prone to cracking and breaking upon handling, for example in the lamination process and when fold forming of the laminate into packages. Besides not being flexible in handling at manufacturing and distribution, laminates including such thick layers of starch would also be capable of absorbing more moisture, which would influence the gas barrier properties negatively.
From WO97/16312 it is known that very thin layers of starch applied on to a core layer may provide gas barrier properties, at least when employed together with an adjacent layer of plastics, which has been united with the starch barrier layer by extrusion coating of the plastics material. Two very thin layers of starch, applied in a quantity of 0.5 and 1 g/m2 respectively, dry weight, on to opposite sides of a core layer of paperboard and each extrusion coated with a layer of plastics, provided an oxygen gas barrier of 289 cm3/m2, per 24 h at 1 atm. Similarly, two layers of starch, applied in a quantity of 1 and 1.5 g/m2 respectively, provided an oxygen gas barrier of 141 cm3/m2, per 24 h at 1 atm. The results obtained were thus, comparable with the gas barrier properties of, for example, a 12 xcexcm thick film of oriented PET, thus representing a xe2x80x9cmedium performance barrierxe2x80x9d material.
The packaging laminate WO97/16312 is, however, merely a medium performance gas barrier material. This means that it may only be used for packaging of liquid food products during short time periods of cool storage. It is not hitherto known in the prior art to produce packaging laminates having high performance gas barrier properties from starch or starch derivative barrier materials. It would be much more desirable to be able to provide a packaging material having sufficient gas barrier properties for long time storage of liquid food products, i.e. for extended shelf life (ESL) at cool storage or even for aseptic storage. Such desirable high performance oxygen gas barrier properties are in the order of about 50 cm3/m2 at 24 h, 1 atm (23xc2x0 C., 50% RH) or better, e.g. up to 30 cm2/m2 at 24h 1 atm, i.e. oxygen gas barrier properties comparable to those of, for example, PVOH, EVOH (ethylene vinylalcohol copolymer) or polyamides (PA) when employed at a thickness of the order of about 5 xcexcm.
We have now established that it is possible in a packaging laminate to obtain high performance oxygen barrier properties from the use of starch and similar materials.
Accordingly the present invention now provides a packaging laminate, preferably having a core of paper or paperboard, having one or more gas barrier layers of starch or a starch derivative providing an oxygen gas barrier property of 50 cm3/m2 at 24 h, 1 atm (23xc2x0 C., 50% RH) or better, said gas barrier layer or layers having a dry coating weight or aggregate coating weight no more than 7 gmxe2x88x922. Preferably the gas barrier layer is deposited on a plastics layer carried by said core layer, and/or having a plastics layer laminated at high temperature thereon. Preferably, oxygen barrier property provided by the starch or starch derivative layer is 40 cm3/m2 at 24 h, 1 atm (23xc2x0 C., 50% RH) or better. More preferably the oxygen barrier property is up to 30 cm3/m2 at 24 h, 1 atm (23xc2x0 C., 50% RH), e.g. 10 cm3/m2 at 24 h, 1 atm (23xc2x0 C., 50% RH) or below.
Preferably, the packaging laminate comprises a layer of plastics polymer, preferably a thermoplastics, e.g. polyethylene, laminated directly with the said gas barrier layer. Most preferably, said polymer is LDPE. Other thermoplastics that may be employed include all other kinds of polyethylene (including LLDPE, ULDPE, VLDPE, M-PE and HDPE), polyproplylene, and polyethyleneterephthalate.
The gas barrier layer is applied at a dry coating weight of up to 7 gmxe2x88x922, e.g. from 0.5 to 5 gmxe2x88x922, more preferably 0.5 to 3 gmxe2x88x922, e. g. from 1.5 to 2 gmxe2x88x922.
We prefer that the gas barrier layer be made entirely from natural materials but it is acceptable to include minor amounts of other polymeric materials which do not interfere with the desired properties. For instance the gas barrier layer may further comprise a minor amount of water soluble or water dispersible polymers having functional hydroxyl groups, e.g. polyvinyl alcohol, and carboxyl group containing polyolefins such as ethylene acrylic acid, or a mixture thereof. The amount of such materials may be from 0 to 30%, e.g. 0 to 20% or 0 to 10% by weight.
We have observed that when polyethylene is applied to a layer of starch at a high temperature, e.g. over 200xc2x0 C., the gas barrier properties of the starch are improved and that under appropriate conditions can be made to reach or move further into a high performance level. One preferred method of obtaining optimal properties is to apply the starch or starch derivative not to a thick core layer as in WO97/16312 but to a separate carrier. Suitably then, the gas barrier layer is carried by a carrier layer of paper or plastics.
When paper is employed it is preferably thin, e.g. said carrier layer may be of paper having a surface weight of from 10 to 25 g/m2. The paper may also be coated beforehand with a layer of plastics.
After application of the starch, the carrier may be combined with a thicker core material so that the packaging laminate comprises a core layer having said carrier layer on one surface side thereof. Said core layer may be of paper, paperboard, foamed polymer or thick polymer and there may be one or more layers including a heat sealing layer on the other surface side of said core layer.
The invention includes a method for producing a packaging laminate having gas barrier properties, which process comprises applying a solution or dispersion of starch or a starch derivative in a liquid vehicle to a surface of a substrate, said substrate preferably comprising a paper or paperboard core layer, and removing said liquid vehicle to deposit said starch or starch derivative on said surface, and optionally applying a layer of plastics to said starch or starch derivative so as to modify the properties of said starch or starch derivative such that the starch or starch derivative provides a gas barrier property, wherein the surface of the substrate is substantially impervious to said liquid vehicle and the starch or starch derivative provides a gas barrier property of 50 cm3/m2 at 24 h, 1 atm (23xc2x0 C., 50% RH) or better.
The degree to which the surface is impervious to liquid may be measured by measuring surface adsorption, e.g. in Cobb units. (xe2x80x98Cobbxe2x80x99=g(water)/m2 adsorbed on to the surface in 60 seconds exposure to liquid water). Adsorption of other liquids could be measured in an analogous method. The method of measuring Cobb adsorption is defined in SCAN P12-64 and in TAPPI T441. The surface adsorption of plastics is generally about 1 Cobb, while a smooth paper surface will generally have an adsorption of about 20 to 30 Cobb. Suitably, for use in the invention the substrate surface should have an adsorption of 50 Cobb or less, preferably an adsorption of 30 Cobb or less, more preferably an adsorption of less than 20 Cobb or most preferably an adsorption of 10 Cobb or less, e.g. less than 5 Cobb.
In an alternative aspect, the invention provides a method for producing a packaging laminate having gas barrier properties, which process comprises applying a solution or dispersion of starch or a starch derivative in a liquid vehicle to a surface of a substrate, said substrate preferably comprising a paper or paperboard core layer, and removing said liquid vehicle to deposit said starch or starch derivative on said surface, and optionally applying a layer of plastics to said starch or starch derivative so as to modify the properties of said starch or starch derivative characterized in that the surface of the substrate has a smoothness of 200 Bendtsen or better and in that the starch or starch derivative provides a gas barrier property of 50 cm3/m2 at 24 h, 1 atm (23xc2x0 C., 50% RH) or better. The method of measuring Bendtsen smoothness is defined in SCAN (Scandinavian Pulp and Paper Norms) P21-67 and in TAPPI UM535.
Where the substrate is plastics or has a plastics surface, such desired smoothness is usually obtained, such as in, for example, a film of plastics, a plastics coated laminate substrate, or a plastics coated paperboard laminate substrate.
One reason why a high barrier property was not achieved in WO97/16312 may be that the paperboard core layer lacked the requisite degree of impermeability so that the aqueous solution of starch which was employed may have penetrated the surface. This might have an adverse action in a number of ways. There may not then be a smooth and unbroken surface to the starch layer because of penetration as such into the paperboard. Alternatively, or additionally, drying of the paperboard to dry the starch layer may cause surface deformation of the paperboard and hence cracking of the starch layer.
The substrate is therefore desirably sufficiently impervious to the liquid vehicle or sufficiently liquid repellent that there is a homogeneous starch layer of even thickness formed upon drying the starch layer.
The paperboard used in WO97/16312 would typically be expected to have had a surface smoothness of 500-600 Bendtsen. This may in itself have been sufficient to prevent the starch layer being smooth and unbroken or from having thin areas providing a path for oxygen transmission.
In order to avoid cracks, punctures or deformations in the starch or starch derivative layer, it is preferred that the surface on to which it is applied is smooth, e.g. that the substrate surface has a smoothness of 200 Bendtsen or better (i.e. less ), e.g. from up to 150 Bendtsen, most preferably about 100 Bendtsen.
The packaging laminate may consist solely of the gas barrier layer and the plastics polymer layer, the substrate being used only in the making of the laminate without being a permanent part of it. Thus, a solution or dispersion of starch or a starch derivative may be applied to a hard substrate plate (such as glass, ceramic or metal) or another form of temporary carrier and dried to form the required layer of starch (or its derivative). Polyethylene or other thermoplastics may be extrusion coated onto the starch (or derivative) layer and the resulting two layer laminate may be stripped off the carrier to provide a plastics film having improved gas barrier properties. Optionally, a second plastics layer may be applied to the opposite side of the starch layer to encapsulate the starch.
More preferably however, the substrate to which the starch or derivative is applied becomes a permanent part of the packaging laminate. Where the finished laminate comprises a core layer of sufficient thickness to provide rigidity to containers made therefrom, e.g. paperboard, it is preferred that the starch or derivative is not applied directly to the core material but rather that it is applied to a carrier material as said substrate and that this is later (before or during the application of said plastics layer) laminated to the core layer, e.g. by extrusion lamination.
Starch for use in the invention may be of any conventional type although certain starches provide better results than others under the conditions we have used. Modified potato starch is preferred, such as Raisamyl 306 (Raisio) which is hypochlorite oxidised. Other acceptable starches include corn starch and derivatives, such as Cerestar 05773, a hydroxypropylated corn starch.
Starch derivatives that are suitable for use in the invention include oxidized starch, cationic starch and hydroxpropylated starch.
It will be understood that when the gas barrier property of the packaging laminates of the invention is referred to as being provided by a particular material, e.g. starch or a starch derivative, this does not exclude the case where the gas barrier property is the result of an interaction between the stated material and an adjacent layer in the laminate, rather than a bulk property of the stated material viewed in isolation.
It may be that a contributing mechanism in the improvement in barrier property noted when polyethylene is applied at a high temperature to a layer of starch comes from penetration of polyethylene molecules into the starch, replacing water in starch crystals. Other polymers producing a similar effect may be used.
Said plastics layer may be applied to said starch or starch derivative by melt extrusion or may be applied as a pre-formed film by hot pressure lamination e.g. with a heated roller. Generally, any technique may be employed in accordance with this preferred embodiment that provides the required modification of the barrier property of the starch.
Preferably said plastics layer is bonded to the starch or starch derivative at a temperature of at least 200xc2x0 C., preferably from 250 to 350xc2x0 C. most preferably from 250 to 330xc2x0 C.
The invention includes a packaging container or package formed using a packaging laminate as described or by a method as described according to the invention.